What signs indicate that someone has a cocaine habit?

Sometimes none: and sometimes many, as with ANY form of chemical dependency. Some people are excellent at hiding it until the very end (and even there many people may not know about it), while with others, it is apparent up front. Changes in personality&hygiene may occur, periods of overactivity with pressured speech (hypomania) and even psychosis may occur. Powder around nose, track marks, burns near mouth/nose.